To be able to guarantee the operability of memory chips over a relatively long period, the memory chips are subjected to artificial aging. If the error rate of memory chips is plotted over their age, then the result is a characteristic curve similar to the shape of a bath tub, i.e. most of the chips are faulty from the outset or become faulty only after an extended time. Artificial aging, generally called burn-in, is carried out in a type of furnace at raised temperature and at raised internal electrical operating voltages.
During artificial aging of the memory chips, said memory chips are operated in a test mode in which, normally, the internal voltage sources for the memory chips are first disconnected and replaced by external voltage sources having higher supplied voltages, and secondly the data which the memory chips output have been inverted. During the test mode, data are written to the memory chips continuously and are read therefrom continuously.
A crucial quality criterion for artificial aging is that the memory chips remain in the test mode throughout the artificial aging process, since otherwise there is no assurance of their being loaded by the raised internal voltages. To be able to single out memory chips which are not in the test mode, activation of the test mode needs to be constantly checked.
Activation of the test mode is normally checked by checking whether a memory chip delivers inverted data, in line with the test mode stipulations. If this his not the case, the memory chip is faulty and/or is assessed as faulty by a tester and can be singled out.
A drawback of checking activation of the test mode using the inverted data is that additional inverters need to be held in the memory chips in order to invert the data, which increases the memory chips' circuit complexity, required chip area, etc.
Another drawback is that, in the memory chip's normal operating mode or normal mode, the required inverters encumber and slow down the data path in the memory chip, since they are always contained in the data path. This is due to the extended delay times through the inverter infrastructure, such as through lines, latches, etc., and to loads, such as capacitive loads, arising as a result of the inverters in the data path.